I have this cool tester (it's called a Block Check by MrGasket) I use whenever I suspect something like that. What it does is to detect the presence of exhaust gas in the radiator or degas tank, by drawing the air in there, through a liquid that has a color indicator in it.
I don't think they're expensive, mine is 30yrs old and still works great, it sure would ease my mind Buy or borrow one to ease yours!
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